Vehicle brakes are often controlled by hydraulic systems, in which a pedal-actuated valve is used to control supply of hydraulic fluid to the brakes. Some vehicles have more than one brake system, for instance front and rear brakes. Hydraulic systems for such vehicles use two valve sections, one for the front brakes and the other for the rear brakes, which allow flow of hydraulic fluid to the front and rear brakes when a brake pedal is pressed by an operator. Prior hydraulic systems often have the two valve sections configured in series which may result in brake pressure bias and/or overshoot. Bias occurs when the brake pressure provided to the two brake systems is not equal; overshoot occurs when the brake pressure exceeds the nominal pressure setting of the pedal valve. Bias and overshoot may have a negative effect on brake and axle component durability.
Many vehicles have a second actuation system for the brakes, which may be used for retarding, anti-skid braking or dynamic brake control. Typically the hydraulic system includes a solenoid-controlled valve for each brake system to provide the second actuation system and additional valves to resolve and determine whether pedal-actuation or solenoid-actuation will control the supply of hydraulic fluid to the brake systems. Inclusion of additional valves may increase bias and overshoot between brake systems, and may increase overall cost of the hydraulic system.